Differential gear mechanism for a drug delivery device and drug delivery device

A drug delivery device comprising:

• a primary drug delivery assembly comprising
• a primary dose dial sleeve configured to move proximally in a helical movement during setting of a dose of a primary medicament contained in a primary reservoir of the primary drug delivery assembly;
• and
• a secondary drug delivery assembly comprising a secondary drive sleeve configured to move in a proximal direction in a helical movement during setting of a dose of a secondary medicament contained in a secondary reservoir of the secondary drug delivery assembly;
• and
• a differential gear mechanism comprising:
• a first toothing;
• a transmission element movable relative to the first toothing in a first axial direction and comprising at least one first gear wheel and at least one second gear wheel, the second gear wheel concentrically arranged with respect to the first gear wheel;
• a second toothing axially constrained to the primary dose dial sleeve;
• a third toothing axially constrained to the secondary drive sleeve;
• and
• a fourth toothing;
• wherein the second toothing, the third toothing and the fourth toothing are moveable relative to the first toothing in the first axial direction wherein the first gear wheel is in meshed engagement with the first toothing and the fourth toothing, and
• wherein the second gear wheel is in meshed engagement with the second toothing and the third toothing.